Ion selective, field effect transistors or ion sensitive sensors are applied for measuring ion concentrations or for measuring special substance concentrations in solutions with different compositions and different conductivities.
The sensitive layers of ion selective, field effect transistors are almost exclusively formed by amorphous layers of e.g. simple metal oxides, as e.g. Ta2O5, Al2O3, TiO2, HfO2, simple metal nitrides or double metal oxide mixtures such as e.g. TaAlO and ZrAlO or combinations of two different amorphous metal oxide layers, which always have SiO2 as base material. A large number of additionally suited materials, which are available for ISFETs as a function of the respective application, are described in the not pre-published patent applications of the assignee, namely in the German patent application DE 102009002060.8, filed on Mar. 31, 2009, and in the International patent application PCT/EP2010/053275, filed on Mar. 15, 2010. All there named options of material compositions are expressly incorporated by reference in the disclosure of the present patent application.
ISFETs are well established examples for sensors having an EIS structure, wherein here the insulator forms the ion sensitive gate insulator of the field effect transistor. ISFETs are widely applied for the continuous detection of concentrations and for pH measurements for environmental monitoring, in industrial automation technology, in the foods industry and in biochemistry and medical technology. The advantages of ISFETs lie in their glassless construction, in their highly precise registration of concentrations, in their fast start-up and in their minimum long time drift combined with an acceptable price/power ratio.
In the case of so-called LAPS—Light Addressable Potentiometric Sensors—by means of a modulated light signal, photoelectrons are produced in the semiconductor material of an EIS structure. The generating of the photoelectrons is dependent on the particular properties of the electrolyte. A basic description of LAPS is given in the article, “Light Addressable Potentiometric Sensor For Biochemical Systems”, Hafeman et al., Science 240 (1988), Pgs. 1182-1185.
In automation technology, especially in process automation technology, ion selective, field effect transistors or ion sensitive sensors are often subjected to wearing sterilization- and/or cleaning-processes. Referenced here are especially SIP—(Sterilization In Process) processes and CIP (Cleaning In Process) processes. In the case of CIP processes, which are applied frequently in the foods industry, for example, for cleaning pipeline systems and which are often absolutely required, the ion selective, field effect transistors or the ion sensitive sensors are typically exposed for a time period of about a half hour to strong acid or base solutions, which have a temperature of about 85° C. For SIP processes, which serve, for example, for sterilizing pipeline systems, the installed sensors are heated for a certain time period to about 130° C. Through these processes, the ion selective, field effect transistors or ion sensitive sensors are unavoidably influenced as regards their functionality and accuracy of measurement: After the CIP or SIP process, is the sensor is often in a state, which deviates from the state, which the sensor had before the CIP or SIP process, and which without knowledge of this change would lead to bad measurements.
WO 2005/073706 A1 discloses an improved gate configuration for an ISFET pH sensor. Especially, here, a sensitive layer of tantalum oxide is applied on an aluminum layer. While the sensitive layer of tantalum oxide assures a high measurement quality, the aluminum layer increases the life of the ISFET pH sensor, since it prevents penetration of the measured liquid into the substrate oxide layer.
Problematic in the case of CIP and SIP processes is that the charging characteristic of an ion selective, field effect transistor or an ion sensitive sensor is changed. This leads to fluctuations in the measurement results, which are not caused by a corresponding change in the composition of the medium to be determined or monitored.